Beta Hydri

Primary of Hatshehogan.

Beta Hydri (β Hyi, β Hydri) is a star in the southern circumpolar constellation of Hydrus. (Note that Hydrus is not the same as Hydra.) With an apparent visual magnitude of 2.8, this is the brightest star the constellation. Based upon parallax measurements the distance to this star is about 24.33 light-years (7.46 parsecs).

This star has about 108% of the mass of the Sun and 181% of the Sun's radius, with more than three times the Sun's luminosity. The spectrum of this star matches a stellar classification of G2 IV, with the luminosity class of 'IV' indicating this is a subgiant star. As such, it is a slightly more evolved star than the Sun, with the supply at its core becoming exhausted. It is the nearest subgiant star to the Sun and one of the oldest stars in the solar neighborhood. This star bears some resemblance to what the Sun may look like in the far distant future, making it an object of interest to astronomers.

At around 150 BC, this star was two degrees away from the southern celestial pole. It is currently the nearest relatively bright star to the southern pole.

In 2002 Endl et al. inferred the possible presence of an unseen companion orbiting Beta Hydri as hinted by radial velocity linear trend with a periodicity exceeding 20 years. A substellar object with minimum mass of 4 Jupiter masses and orbital separation of roughly 8 AUs could explain the observed trend. If confirmed, it would be a true Jupiter-analogue, though 4 times more massive. So far no planetary/substellar object has been certainly detected.